The present invention relates to imaging systems, and relates in particular to maskless lithography systems.
In a maskless lithography system, an array of diffractive lenses such as Fresnel zone plates may be used to form an array of tightly focused spots on a photosensitive layer that is on top of a substrate. For example, U.S. Pat. No. 5,900,637 discloses a maskless lithograph system and method that employs a multiplexed array of Fresnel zone plates. The light incident on each diffractive lens may be controlled, for example, by one pixel of a spatial light modulator. The spatial light modulator for use in such a system should provide a high refresh rate, be able to operate at short wavelengths such as under 200 nm, and be able to perform grayscaling or intensity modulation in real time.
One commercially available spatial light modulator that may satisfy the above requirements is the grating light valve (GLV) spatial light modulator made by Silicon Light Machines of Sunnyvale, Calif. The GLV consists of a linear array of pixels, and each pixel consists of six metallic ribbons that form a diffraction grating. Alternate ribbons may be moved by electrostatic actuation to provide either a reflective surface or a grating.
In a maskless lithography system employing a GLV, one pixel of a GLV 10 addresses light into one zone plate 12 of a zone-plate-array 14 as shown in FIG. 1. The zone plate 12 then focuses the light onto a writing surface 16 of a substrate 18 that is supported by a stage 20. When actuated, light from the +1 order of the pixel on the GLV 10 is directed toward the zone-pate 12. When the pixel is not actuated, light from the GLV 10 is directed into the 0 order (as shown at 24 for the surrounding pixels on the GLV 10). Real-time grayscaling may be achieved by varying the depth to which the ribbons of the GLV 10 are pulled.
Since the GLV is a linear device, at any instant of exposure the focused spots lie on a line having a length L on the writing surface 16 of the substrate 18. Although the lens array 14 and writing surface 16 are shown as including five lens and five associated pixels only for diagrammatic purposes, the line may have a length of for example, 1088 pixels in certain applications. In semiconductor lithography for example, a writing line may be sequentially imaged over the two-dimensional writing surface in, for example, a serpentine fashion. The substrate in such a semiconductor lithography system may include a silicon wafer and the focal distances may be on the order of nanometers. It may be difficult however, in certain semiconductor lithography systems to provide a line area on a wafer that is flat to within the depth-of-focus distance of the lens array. In such cases, it may be necessary to reduce the length of the line, which increases the time required to image the writing surface 16. Further, certain spatial light modulators are capable of modulating far more pixels than may be imaged onto the writing surface as a line.
There is a need therefore, for an imaging system that more efficiently and economically provides maskless lithography using a spatial light valve that provides a line of modulated illumination.